Increasing the efficiency of membrane bioreactors

Paul Boughton

Increased recycling of water that has been contaminated during manufacturing processes would help to significantly reduce industry's carbon footprint, according to research by De Montfort University (DMU) in Leicester, England.

Researchers have found a way to boost the energy efficiency of a method of recycling industrial wastewater which, if adopted by companies, would help them to reduce the overall amount of water they buy in.

The study marks the first time that the efficiency of membrane bioreactor (MBR) plants has been assessed using computer-modelling techniques. Academics leading the project believe initial results show plants can be made between 10-15 per cent cheaper to operate.

Industrial wastewater is water which has been contaminated while being used for a specific purpose, for example in the brewing process during the manufacture of alcohol, or in the cleaning of ingredients used for food products.

Biological materials

Contaminants typically include biological materials or industrial by-products and the wastewater is usually thrown away.

Parneet Paul, a senior research fellow within Process Control - Water Software Systems (PC-WSS) and the project coordinator, said: "If the water is recycled, it can be reused, thereby reducing a company's water utility costs and helping to reduce their energy footprint.

"If this wastewater can be fully recovered in an efficient manner it not only has cost implications for industry as a whole, but could also be a way for industry to meet its targets to reduce its green house gas emissions.

Many industries currently do not use these advanced wastewater treatment systems for full water recovery since their performance has not been fully analysed to date by using computer-modelling techniques."

The three-year, £402 000 project was funded by the UK government's Technology Strategy Board as part of its technology programme.

Modelling techniques

The aim of the project was to use the latest modelling techniques coupled with measurements taken from electronic sensors and using biochemical laboratory testing, to make these advanced wastewater treatment plants more energy efficient and cost effective for industrial companies.

These advanced methods which use ultra-filtration membranes produce wastewater of excellent quality which is suitable for recycling and could potentially save industrial companies many thousands of pounds in water utility costs.

Membrane bioreactors consist of a biological reactor containing a very high concentration of selective bacterial micro-organisms which 'digest' organic contaminants generated from the manufacturing processes.

Inorganic pollutants

The wastewater is then pumped through an ultra-filtration membrane to remove inorganic pollutants. It is then clean enough to be reused as a secondary supply of water, or can be further purified using finer membranes and disinfection procedures.

The main operational energy costs are due to aeration of the micro-organisms in the bioreactor and having to pump liquid through the membranes at high pressures.

During the project, computer models were used to maximise the biological aeration processes which inject oxygen into the bioreactor and to minimise membrane fouling caused by the accumulation of solids during the filtration process which can reduce clean water production rates and potentially damage the expensive membrane modules.

"Using membrane bioreactors will help to improve upon the sustainable capabilities of any industries that use large volumes of clean water," Paul added (Fig.1).

"We hope that the results will encourage industry to take advantage of these treatment systems as it could help them to be more environmentally friendly and reduce their operating costs at the same time."

Research group

The project was carried out by DMU's PC-WSS research group in collaboration with industrial partners, including: Aquabio, a SME company specialising in novel and advanced wastewater treatment methods especially using membranes (Fig. 2); ITT Sanitaire (UK), a subsidiary of a large US company with a board interest in all aspects of the water engineering sector; and Northern Ireland Water, a government-owned water utility company.

Paul added: "Our project objective was to use computer models to better predict plant performance, and therefore increase the take up of this new technology.

"Recent technological developments and a continuing radical change in attitude towards waste generation mean that new, advanced solutions are increasingly becoming available for industrial clientele.

"UK research has to lead the way in introducing these new systems to a wider industrial audience."

Meanwhile the Butler Drive water reclamation facility (WRF) in Peoria, Arizona, has claimed its second national project-of-the-year award when the US WateReuse Association gave the innovative facility its top project award in the large-project category.

The association's annual awards recognise projects and individuals that advance the beneficial and efficient use of water resources through education, sound science and technology using reclamation, recycling, reuse or desalination for the benefit of the public and the environment.

In 2009, the Black & Veatch-designed facility was named as national project of the year in the environment (more than US$75 million) category by the American Public Works Association (APWA).

Peoria is located in an arid desert environment, where ensuring long-term water resources is critical. The Butler Drive WRF, which currently produces approximately 8m gallons/day of water, began operating in July 2008. It features the largest MBR in North America.

Use of membrane bioreactor technology to produce high-quality effluent for aquifer recharge allows the growing city to earn water credits and therefore extract the equivalent amount of water from the aquifer to augment its supply and meet future needs.

With the future addition of several membrane cassettes, the facility will ultimately produce up to 13m gallons/d of water for aquifer recharge.

Black & Veatch provided design and construction-phase services for the US$135 million facility. The full treatment process includes fine screens, activated sludge with biological nitrogen removal, and UV disinfection as well as membrane filtration.

Low-profile structures and effective odour control are designed to make the WRF a good neighbour.

Wastewater treatment

MBRs are also finding their way into smaller wastewater treatment processes. Take for example the 1000-acre Laurel Cove Golf & Country Club in middle Tennesee, USA, which is being developed into both a golf course and 800-strong residential community.

The decision makers had expressed a strong desire to incorporate the best available technology to allow for water conservation leading to Bord na Móna being awarded a contract for the design and supply of a PuraM MBR wastewater treatment plant.

A tight schedule was required as local regulation required that the wastewater plant had to be constructed prior to building the homes.

The system installed at Laurel Cove includes 18 flat-plate PuraM membrane cassettes designed to treat a phase one flow of 250 000 gallons/d and to achieve a final effluent quality of 10mg/l BOD: 20mg/l TSS: 10mg/l TN.

The design allows the final effluent to be used as a supply for irrigation water on the development's Greg Norman-designed golf course.

The PuraM wastewater treatment plant is comprised of two treatment streams each designed to handle up to 125 000 gallons/d. Each stream has an anoxic/equalisation tank and PuraM MBR tank. Consideration was made in the design to allow for upgrading the system in future phases for flows up to 1 000 000 gallons/d.

Reduced operational input

The PuraM MBR was chosen because of its design that is particularly suitable for the decentralised market with an emphasis on reduced operational input, ease of maintenance and less complexity than comparable systems.

The flat-plate ultrafiltration membrane treatment technology is assembled into stainless steel membrane cassettes that consist of an integral dedicated air diffuser assembly that eliminates the need for back-pulsing or frequent chemical cleaning.

The system does not require permeate pumps or any site-installed chemical dosing system while achieving a typical time between recovery cleans of approximately 12 months, significantly better than comparable water reuse systems

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